The controlled extraction of heat fluxes of order 1000 W/cm.sup.2 remains a challenge in many fields, including steel processing, nuclear reactor safety, and high-temperature materials synthesis. The field of materials synthesis is of particular interest with respect to the production of diamond.
The commercial production of synthetic diamonds began in the 1950's with the development of pressure cells capable of achieving combinations of temperature and pressure of sufficient duration to turn common forms of carbon into diamond. Such pressure-based techniques have been refined over the years to the point where high pressure is the basis for the production of considerable quantities of diamond. Still, these techniques are very demanding of the equipment they employ, requiring sustained pressures of several millions of atmospheres to form diamond.
A somewhat contrary approach is known as Chemical Vapor Deposition (commonly abbreviated as CVD). This approach is based on the realization that diamonds can be formed in low-pressure conditions from a heated gas comprising isolated carbon-containing radicals. Under suitable conditions of temperature and with a proper substrate, these species can be made to deposit carbon in the form of a diamond film.
One of the technical problems attendant to practicing this technique lies in properly controlling the high temperatures of the substrate--generally, a mandrel surface--on which the diamond film forms. Briefly, in flame based diamond CVD, a premixed flame is created in a stagnation point flow against a surface on which diamond film growth is desired. The reactants--typically acetylene, hydrogen, and oxygen--create a high flame temperature of approximately 3000.degree. K. It will be appreciated that the production of this flame is accompanied by high rates of heat and mass transport to the deposition surface (i.e., the substrate) of the mandrel. In other words, in CVD the flux of energetic molecular species to the surface is accompanied by a large flux of heat which must be removed from the mandrel if control of the growth process is to be maintained. For low-pressure (i.e., sub-atmospheric) flame CVD, the heat flux is sufficiently low that conventional methods of convective and/or radiative cooling may be use. At higher pressure, where diamond growth rates increase by an order of magnitude, heat fluxes of order (i.e., within a factor of ten of) 1000 W/cm.sup.2 must be extracted.
The situation is further complicated by the need to hold the deposition surface temperature to a tight tolerance somewhere within the optimal diamond growth range (.about.1200.degree. K), and the requirement that heat extraction be made in a one-dimensional fashion to preserve the uniform boundary condition on the flame. Additional problems that must be taken into consideration include compensating for the discontinuity of flux that arises at the boundary of the finite-area flame, providing for real time control and tuning of the heat extraction process, and compensating for process fluctuations. It would also be desirable to provide scalability to accommodate projected commercially sized mandrels (typically 6-8 inches in diameter).
The heat flux created by the flame must be removed from the mandrel if the temperature of the mandrel diamond deposition surface is to be maintained within acceptable limits for the rapid growth of high quality diamond film. Excessive deviation of the temperature of the substrate from the optimal level will result in a decline in the quality of the diamond film, a diminution in its growth rate, or both. Given the complexity of the combustion process that gives rise to the heat and mass flows, it is far preferable to control this heat directly at the mandrel by removing heat from the mandrel (and hence the deposition surface) than by attempting to modify the kinetics of the flame itself.
So-called pool-boiling has been employed to control mandrel temperature in a variety of applications. In pool boiling, a portion of a mandrel is submersed in water to provide heat extraction. Pool boiling provides efficient heat transfer via phase change of the coolant where the heat fluxes through the mandrel are moderately high and the temperature of the surface submerged in water exceeds the boiling point.
Unfortunately, the heat flux capabilities of pool boiling fall just short of those required for high heat flux applications such as diamond synthesis. This is because of the so-called critical heat flux or burn-out problem of boiling heat transfer. At sufficiently great levels of heat flux, the coolant immediately adjacent the hot surface boils so rapidly that a continuous vapor film is formed adjacent to the surface, which inhibits subsequent wetting of the surface, and impedes further heat transfer (the so-called Leidenfrost effect). Forced convective or pressurized boiling provide some extension of cooling capabilities to higher heat fluxes, but this is limited to less than an order of magnitude improvement. Both pool boiling and forced convective boiling are ultimately limited by the burnout phenomenon.
Dilute spray cooling has been employed to circumvent this problem by placing the cooling fluid on the surface in discrete quanta, i.e., droplets, separated by many diameters in space and surface-interaction periods in time. This precludes the possibility of forming a contiguous vapor film (and burnout), since only small portions of the surface are in contact with the water at any instant and only small, isolated regions of vapor are formed. Using spray cooling at low surface superheat temperatures, heat flux capacities in excess of 1000 W/cm.sup.2 have been demonstrated in several studies, provided the temperature on the surface being sprayed remains fairly close to the boiling point of the water (and thus below the Leidenfrost temperature). This method of cooling has been studied extensively both out of theoretical interest and for the possibility of application to the electronics and the nuclear power industries.
Less attention has been paid to the problem of spray cooling at high surface superheat temperatures. This is due in large part because many processes cannot tolerate high surface temperatures, and also because onset of the Leidenfrost phenomenon (where the spray droplets no longer wet the surface) causes cooling effectiveness to decrease by an order of magnitude from the low superheat case.
One approach to providing a spray cooling solution to this problem calls for the heat extraction surface of the mandrel to be uniformly coated with a cooling spray with sufficient capacity to ensure that the surface remain near the saturation temperature of the coolant (generally water). This uniform surface temperature (or isothermal boundary condition) guarantees that the heat extraction is one-dimensional (given a one-dimensional heat input) and that sufficient heat extraction can be maintained over the required range of operating conditions without adjustment of the spray. Unfortunately, these desirable characteristics come at a price--inflexibility in control of the deposition surface temperature.
Since, in such an approach, the temperature of the cooling surface is generally fixed near the saturation condition by the phase change of the droplets, and the heat flux into the mandrel is imposed by the flames, the only way to achieve a desired deposition surface temperature has been by varying the thermal resistance of the mandrel itself. This has been accomplished by bolting together a series of disks to achieve the required temperature rise from the isothermal boundary to the deposition surface. Although this is a workable solution under some conditions, this approach suffers several drawbacks:
Any adjustment or change in flame conditions (i.e., imposed heat flux) requires a change in mandrel configuration to hold the temperature constant. The correct value of resistance must be determined by a series of trial and error experiments which are time consuming and expensive to accomplish. Also since these changes cannot be made on line, the available level of flexibility in control over the process is poor, and real-time control is impossible. Process fluctuations which cause the heat flux to vary (e.g., changing the reactant composition or flame/mandrel spacing) will cause the deposition surface temperature to deviate from the desired set point. PA1 Since the cooling surface is isothermal, uniform temperature at the deposition surface will only result if the heat flux through the mandrel is uniform, that is, if the sides of the mandrel are effectively adiabatic and the flame is uniform over the mandrel surface. If either of these conditions is not met, the deposition surface temperature cannot be made uniform using this method. PA1 This method precludes the provision of spatial profiling of the heat extraction from the mandrel. Variations of the heat flux profile into the mandrel cannot be compensated in this design. PA1 1) permit the extraction of large heat fluxes from the mandrel, typically on the order of 1000 Watts/cm.sup.2, and particularly in the range of 200-400 Watts/cm.sup.2 (which is of interest in current commercial processes); PA1 2) allow for control over the deposition surface temperature of the mandrel independently of any variations which may arise in the flame itself; PA1 3) provide a spatially uniform surface temperature across the face of the mandrel, where such is desired; and PA1 4) be scalable to allow cooling of mandrels larger than those that are currently being used (e.g., mandrels having a substrate area as large as those employed in the semiconductor industry, where diamond films may find significant application).
These limitations could be overcome if it were possible to carry out the spray cooling process without being tied to the isothermal boundary condition inherent in phase-change cooling. In general, there remains a need for a means of controlling the temperature at the deposition side of a heated mandrel that avoids the problems present in known approaches to cooling a high heat flux surface. In particular, there remains a need for a cooling process that would: